The graph is from a new working paper by Hunt et al (September 2012). The data is from the US and the authors explain (p. 15):
The most important determinants of the gender patenting rate gap among S&E [Science & Engineering] degree holders are women’s underrepresentation in patent–intensive fields of study, especially electrical and mechanical engineering, and in patent–intensive job tasks, especially development and design. Women’s lower share of doctoral degrees plays a minor role.
Hunt (2010) demonstrated that women trained as engineers disproportionately leave engineering jobs due to dissatisfaction with pay and promotion (p. 16).
[I wonder if there is more to this story, such as having children affecting more women then men; but again, we see a large increase in female labor participation in recent years].